The burning of solids depends on heating the burnable materials the combustion point. Evaporation of the water content, i.e. the drying, depending on the moisture content of the burnable material, takes place simultaneously with the heating of the burnable material. Since the amount of heat necessary for evaporation of the water is higher than necessary for the combustion of the solids, the most critical phase of the process taking place in the furnace is the drying, especially in the case of burnable materials with high moisture content.
This problem appears mostly during the burning of agricultural plant refuse and, residues, namely they contain a large amount of moisture during harvesting.
These residues are characterized also by the fact that even in a normally air-dry condition, they contain a relatively large amount of moisture, consequently their utilization by burning represents an increased problem, while at the same time the residual products with high moisture content still existing from the harvesting soon begin to rot and become unsuitable for burning.
The furnaces still used for burning agricultural residues essentially apply the methods known for coal burning. The product fed from the hopper onto the grate is dried and brought to the burning point at the front part of the grate by conventional methods used for coal burning, with a socalled combustion crown, and with the reradiation effect of the flame developed in the burning phase. These methods are suitable for burning the air-dried agricultural residues only to a limited extent, since they are able to function only with burnable material of a maximum of 20-25% moisture content. One of the several reasons for this is that when burning agricultural residues, the temperature of the furnace chamber is significantly lower than in the case of coal burning, as a result of which the temperature of the combustion crown is also much lower. Since the heat flow transmitted by radiation is in proportion to the fourth power of the absolute temperature of the combustion crown, the traditional combustion crowns are not capable of drying burnab1e material containing large amounts of moisture.
For this reason burning most parts of the plant residue in freshly harvested condition, as for instance corn stalks, could not be realized.